Congestive heart failure (CHF) is defined generally as the inability of the heart to deliver enough blood to the peripheral tissues to meet metabolic demands. Frequently CHF is manifested by left heart dysfunction, but it can have a variety of sources. For example, CHF patients may have any one of several different conduction defects. The natural electrical activation system through the heart involves sequential events starting with the sino-atrial (SA) node, and continuing through the atrial conduction pathways of Bachmann's bundle and internodal tracts at the atrial level, followed by the atrio-ventricular (AV) node, Common Bundle of His, right and left bundle branches, and final distribution to the distal myocardial terminals via the Purkinje fiber network. A common type of intra-atrial conduction defect is known as intra-atrial block (IAB), a condition where the atrial activation is delayed in getting from the right atrium to the left atrium. In left bundle branch block (LBBB) and right bundle branch block (RBBB), the activation signals are not conducted in a normal fashion along the right or left bundle branches respectively. Thus, in a patient with bundle branch block, the activation of the ventricle is slowed, and the QRS is seen to widen due to the increased time for the activation to traverse the conduction path.
CHF manifested by such conduction defects and/or other cardiomyopathies are the object of considerable research into treatments for improving cardiac output. For example, drug companies have recognized CHF as a market opportunity, and are conducting extensive clinical studies organized to test the outcome of newly developed drugs in terms of improving cardiac performance in these patients. Likewise, it is known generally that four-chamber cardiac pacing is feasible, and can provide significant improvement for patients having left atrial-ventricular dysfunction, or other forms of cardiac heart failure. While there has been relatively little commercialization of four-chamber pacing, the hypothesis remains that cardiac pump function can clearly be improved by such pacing.
The benefits of four-chamber pacing generally have been disclosed and published in the literature. Cazeau et al., PACE, Vol. 17, November 1994, Part II, pp. 1974-1979, disclose investigations leading to the conclusion that four-chamber pacing is feasible, and that in patients with evidence of interventricular dyssynchrony, a better mechanical activation process can be obtained by resynchronizing depolarization of the right and left ventricles, and optimizing the AV sequence on both sides of the heart. In the patent literature, U.S. Pat. No. 4,928,688 is representative of a system for simultaneous left ventricular (LV) and right ventricular (RV) pacing; natural ventricular depolarizations are sensed in both chambers, if one chamber contracts but the other one does not within a window of up to 5-10 ms, then the non-contracting ventricular chamber is paced.
In addition to the above-mentioned disclosures concerning the advantages of substantially simultaneous or synchronous pacing of the two ventricles, it is known that there is an advantage to synchronous pacing of the left atrium and the right atrium for patients with IAB, inter-atrial block. In a normal heart, atrial activation initiates with the SA node, located in the right atrial wall. In a patient with IAB, the activation is slow being transferred over to the left atrium, and as a result the left atrium may be triggered to contract up to 90 ms later than the right atrium. It can be seen that if contractions in the left ventricle and the right ventricle are about the same time, then left AV synchrony is way off, with the left ventricle not having adequate time to fill up. The advantage of synchronous pacing of the two atria for patients with IAB is disclosed at AHA 1991, Abstract from 64th Scientific Sessions, "Simultaneous Dual Atrium Pacing in High Degree Inter-Atrial Blocks: Hemodynamic Results", Daubert et al., No. 1804. Further, it is known that patients with IAB are susceptible to retrograde activation of the left atrium, with resulting atrial tachycardia. Atrial resynchronization through pacing of the atria can be effective in treating the situation. PACE, Vol. 14, April 1991, Part II, p. 648, "Prevention of Atrial Tachyarrythmias Related to Inter-Atrial Block By Permanent Atrial Resynchronization", Mabo et al., No. 122. For patients with this condition, a criterion for pacing is to deliver a left atrial stimulus before the natural depolarization arrives in the left atrium.
In view of the published literature, it is observed that in CHF patients improved pump function can be achieved by increasing the filling time of the left ventricle, i.e., improving the left AV delay, and specifically the left heart mechanical AV delay (MAVD); decreasing mitral valve regurgitation, (back flow of blood through the nearly closed valve) by triggering contraction of the left ventricle when and as it becomes filled; and normalizing the left ventricular activation pattern, i.e., the time sequence of left atrial contraction relative to right atrial contraction. More specifically, for a cardiac pacing system used for treating a CHF patient, the aim is to capture the left atrium; optimize the left AV delay so as to properly fill the left ventricle and provide a more normal AV delay; and activate the left ventricle as much as possible in accordance with the natural propagation path of a healthy left heart. Particularly, left ventricular timing with respect to the left atrial contraction is crucial for improving cardiac output. The mechanical closure point of the left, or mitral valve, is a crucial moment which needs to be adjusted by programming of the left AV delay. Correct programming of this variable is key for optimizing the filling of the left ventricle, and optimizing ejection fraction, or cardiac output (CO).
An observation which is important to this invention is that the exact timing of mechanical events are important for properly controlling pacing so as to optimize left ventricular output. Specifically, it is known that actual contraction of one ventricular chamber before the other has the effect of moving the septum so as to impair full contraction in the later activated chamber. Thus, while concurrent or simultaneous pacing of the left and right ventricle may achieve a significant improvement for CHF patients, it is an aim of this invention to provide for pacing of the two ventricles in such a manner that the actual mechanical contraction of the left ventricle, with the consequent closing of the valve, occurs in a desired time relationship with respect to the mechanical contraction of the right ventricle and closing of the right value. For example, if conduction paths in the left ventricle are impaired, delivering a pacing stimulus to the left ventricle at precisely the same time as to the right ventricle may nonetheless result in left ventricular contraction being slightly delayed with respect to the right ventricular contraction. As a consequence, it is important for this invention to provide a technique for measurement of mechanical events, such as a mechanical closure point of each of the ventricles, so as to be able to accurately program the sequence of pacing to achieve the desired dual ventricular pacing which optimizes ejection fraction, or cardiac output, for the individual patient.
In view of the above-noted importance of measuring mechanical events., such as mitral or tricuspid valve closure, and the importance of measuring cardiac output, it is necessary for the pacing system of this invention to employ sensors which can provide this information. It is known to use impedance sensors in pacing systems, for obtaining information concerning cardiac function. For example, reference is made to U.S. Pat. No. 5,501,702, incorporated herein by reference, which discloses making impedance measurements from different electrode combinations. In such system, a plurality of pace/sense electrodes are disposed at respective locations, and different impedance measurements are made on a time/multiplexing basis. As set forth in the referenced patent, the measurement of the impedance present between two or more sensing locations is referred to "rheography." A rheographic, or impedance measurement involves delivering a constant current pulse between two "source" electrodes, such that the current is conducted through some region of the patient's tissue, and then measuring the voltage differential between two "recording" electrodes to determine the impedance therebetween, the voltage differential arising from the conduction of the current pulse through the tissue or fluid between the two recording electrodes. The referenced patent discloses using rheography for measuring changes in the patient's thoracic cavity; respiration rate; pre-ejection interval; stroke volume; and heart tissue contractility. It is also known to use this technique of four point impedance measurements, applied thoracically, for measuring small impedance changes during the cardiac cycle, and extracting the first time derivative of the impedance change, dZ/dt. It has been found that a substantially linear relation exists between peak dZ/dt and peak cardiac ejection rate, providing the basis for obtaining a measure of cardiac output. See also U.S. Pat. No. 4,303,075, disclosing a system for measuring impedance between a pair of electrodes connected to or in proximity with the heart, and processing the variations of sensed impedance to develop a measure of stroke volume. The AV delay is then adjusted in an effort to maximize the stroke volume.
Given the demonstrated feasibility of four-chamber cardiac pacing, and the availability of techniques for sensing natural cardiac signals and mechanical events, there nonetheless remains a need for developing a system which is adapted to the cardiac condition of a patient with CHF, so as to provide pacing sequences which are tuned for improving cardiac output, and in particular for improving left heart function. It is a premise of this invention that such a system is founded upon accurate measurements of mechanical events, and use of the timing of such mechanical events to control and program pacing sequences.